1. Field of the Invention
The present invention relates to a deriving device and a driving method of an organic thin film electroluminescent (EL) display. In particular, the present invention relates to a driving device and a driving method of an organic thin film EL display with reduced power consumption.
2. Description of the Related Art
An example of conventional methods for driving an organic thin film EL display is described in Japanese Patent Laid-Open Publication No. Hei 9-232074. FIG. 1 is a circuit diagram of the prior art showing an example of a conventional configuration where data electrodes Xi and scanning electrodes Yj arrayed in a matrix are passively driven, A blanking period is provided between display periods. All the switching circuits 9i, 7j are switched to the ground side in response to a blanking pulse transmitted during this period. As a result, residual electric charges accumulated in all the data lines are discharged. In FIG. 1, reference numeral 2 is an image memory, reference numeral 8 is a driving circuit, reference numeral 4 is an organic thin film EL display, reference numeral 5 is a scanning circuit, reference numeral 51 is a sift register, and reference numeral 6j is an OR circuit.
A pixel P(i, j) is taken for example here. If a scanning electrode Yj to which this pixel P(i, j) belongs is selected, that is, the pixel P(i, j) is in a turned-off state during a display period Tj, a reverse bias is applied to the parallel capacitors of all the pixels P(i, 1) to P(i, j-1) and P(i, j+1) to P(i, n) belonging to a data electrode Xi except for the pixel P(i, j). If a shift is made to the next display period (T(J+1) in this state and pixel P(i, J+1) is turned on, current from a current source circuit 8i connected to the data electrode Xi is first used to cancel charge of the aforementioned reverse-biased parallel capacitors. Consequently, a long delay develops before the pixel P(i, j+1) actually starts emitting light, and thereby a large-capacitance display is not enabled. Thus, a certain effect can be made by providing a blanking period tj between the display period Tj and the display period T(j+1) and applying the data electrode Xi to the ground potential during this blanking period to cancel the charge of the reverse-biased parallel capacitor of the pixel P(i, 1) to P(i, Jxe2x88x921) and P(i, j+1) to P(i, n).
However, if the pixel P(i, j) is in a turned-on state during the display period Tj, all the pixels P(i, 1) to P(i, J=1) and P(i, j+1) to P(i, n) belonging to the data electrode Xi except for the pixel P(i, j) are almost zero-biased. Since the parallel capacitor of the pixel P(i, j) is forward-biased, applying the data electrode Xi to the ground potential during the blanking period tj is not only almost useless, but also electric charges in the forward-biased parallel capacitor of the pixel P(i, j) are wasted.
An object of the present invention is to provide a driving device and a driving method of an organic thin film EL display with power consumption reduced by a configuration where electric charge accumulated in a display element are used to assist a display element to emit light during the next display period.
A driving device of an organic thin film EL display according to a first aspect of the present invention, display elements composed of organic thin film EL light-emitting elements are connected to respective intersections of data electrodes and scanning electrodes arrayed in a matrix. While the scanning electrode is scanned at predetermined periods, the display element emits light in response to a signal applied to the data electrode in synchronization with the scanning. The driving apparatus has a comparator comparing a signal voltage applied to a display element on a predetermined data electrode and on a scanning electrode for the current display period and a signal voltage applied to the display element on the data electrode and on the scanning electrode for the next display period. The driving device also has a controller controlling a discharge of residual electric charges from the data electrode on the currently displaying scanning electrode during a blanking period immediately before the next display period depending on the comparison result by the comparator.
In a driving device of an organic thin film EL display according to the second aspect of the present invention, display elements composed of organic thin film EL light-emitting elements are connected to respective intersections of data electrodes and scanning electrodes arrayed in a matrix. While the scanning electrode is scanned at predetermined periods, the display element emits light in response to a signal applied to the data electrode in synchronization with the scanning. The driving device has a comparator comparing a signal voltage of the display element on a predetermined data electrode and on the scanning electrode for the current display period and a signal voltage of the display element on this data electrode and on the scanning electrode for the next display period. The driving device also has a controller controlling a quantity of residual electric charges discharged from the data electrode on the currently displaying scanning electrode during a blanking period immediately before the next display period depending on the comparison result by the comparator.
If an image signal voltage S(i, j) for the current display period is larger than an image signal voltage S(i, j+1) for the next display period, the controller controls the data electrode on the currently displaying scanning electrode so that residual electric charges are discharged during the blanking period immediately before the next display period. If an image signal voltage S(i, j) for the current display period is equal to or less than an image signal voltage S(i, j+1) for the next display period, the controller controls the data electrode so that the residual electric charges are not discharged.
Further, the controller also controls a discharge circuit which holds the data electrode at the ground level, for example.
Also provided is an image memory having a memory capacity at least enough for 2xc3x97m (m: the number of data electrodes). The signal voltage applied to each data electrode on the currently displaying scanning electrode for a display period and the signal voltage applied to the data electrode on the scanning electrode for the next display period are stored in this image memory so that the comparator can compare the data in the image memory.
The driving device of an organic film EL display also has the same number of discharge circuits as, for example, the number of data electrodes (m).
In the driving method of an organic thin film EL display according to the present invention, display elements composed of organic thin film EL light-emitting elements can connected to respective intersections of data electrodes and scanning electrodes arrayed in a matrix. While the scanning electrode is canned at predetermined periods, the display element emits light in response to a signal applied to the data electrode in synchronization with the scanning. The driving method has steps of comparing a signal voltage applied to the display element on a predetermined data electrode on the scanning electrode for the current display period and a signal voltage applied to the display element on this data electrode and on the scanning electrode for the next display period; and controlling the data electrode so as to be in the discharge state during the blanking period immediately before the display period when the signal voltage applied to the display element on the predetermined data electrode on the scanning electrode for the current display period is larger than the signal voltage applied to the display element on the data electrode for the next display period or controlling the data electrode so as not to be in the discharge state otherwise.
According to the driving device and the driving method of an organic thin film EL display of the present invention, residual electric charges which are conventionally discharged uniformly from all the data electrodes during the blanking period are discharged individually from each data electrode. That is, since residual electric charges do not need to be discharged from a data electrode during the blanking period if the signal voltage for the current display period is not larger than the signal voltage for the next display period, a wasted outflow of electric charges can be prevented by detecting such an electrode. Thus, the first effect of the present invention is electric power saving. It is particularly effective to a display pattern such that all of display elements (pixels) are turned on or the like, where signal voltage applied to each data electrode does not decrease.
According to the present invention, the second effect of the present invention is the improvement of responsiveness when a pixel emits light and the improvement of brightness since residual electric charges which are not discharged during the blanking period are contributed to the charge of the parallel capacitor of a pixel which should emit light during the next display period.
The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.